Added changes from new upstream version

The upstream guys released some performance improvements and other changes, especially when writing out object.

Their changelog:

    Version 4.06, 14 May 2013 
        Added support for Javascript type comments, i.e. // and /* */ 
        Added json_spirit_writer_options.h to CMake install
        Improved error message on attempt to extract the wrong type of data from a value
        Correct bug that reduced write performance 
        Added always_escape_nonascii writer flag